1. Field of the Invention
The present invention relates to an active matrix reflective liquid crystal display device with switching elements such as thin film transistors (hereinafter simply referred to as "TFTs"). More specifically, the present invention relates to a reflective liquid crystal display device characterized by the shapes of its scanning lines, signal lines and pixel electrodes, and a method for producing the liquid crystal display device.
2. Description of the Related Art
Reflective liquid crystal display devices perform a display function by reflecting externally incident light. Since the reflective liquid crystal display devices require low consumption power and are able to provide thin displays for lightweight products, they are recently receiving much attention, for example, for use in displays of portable information terminals such as PDAs (Personal Digital Assistants).
Reflective plates of the reflective liquid crystal display device, which also serve as pixel electrodes, are flat. The flat mirror surfaces of the reflective plates may undesirably reflect objects close to the display, may cause the display to be easily affected by external scattered light, or may cause nonuniform wavelength characteristics of the reflected light due to diffraction or interference which results in coloring of the reflected light, thereby deteriorating the display quality of the device. In order to enhance the display quality of the device under the sunlight or under any kind of indoor light, Japanese Laid-Open Publication No. 7-159776 discloses a technique for providing reflective plates with uneven surfaces.
FIG. 12 is a partial plan view showing an active matrix substrate used in the reflective liquid crystal display device disclosed in the above-mentioned publication.
Referring to FIG. 12, the active matrix substrate includes a plurality of pixel electrodes 54 made of a metal material with a high reflectance provided in a matrix on a base substrate 60 (FIG. 13). Gate lines 52 and source lines 53 run in an intersecting manner so as to surround each of the pixel electrodes 54. At each intersection of the gate lines 52 and the source lines 53, a TFT 51 is provided as a switching element connected to the pixel electrode 54. Each gate electrode 62 (FIG. 13) of each TFT 51 is connected to the corresponding gate lines 52 so that the TFTs 51 are driven and controlled by signals input to the gate electrodes 62 (FIG. 13). Each source electrode 63 (FIG. 13) of each TFT 51 is connected to the corresponding source line 53 so that data signals input to the source electrodes 63 (FIG. 13) are applied to the pixel electrodes 54 via the TFTs 51. Lattice-like storage capacitor lines 55 (hereinafter, simply referred to as Cs lines 55) are disposed beneath the pixel electrodes 54 via a gate insulating film 57 (FIG. 13), thereby forming storage capacitors. The Cs lines 55 are provided with holes 55a, whereby the surfaces of the pixel electrodes 54 are made uneven.
FIG. 13 is a cross-sectional view showing one pixel portion of the reflective liquid crystal display device using the active matrix substrate shown in FIG. 12, taken along line E-E'.
With reference to FIG. 13, the gate electrode 62 protruding from the gate line 52 (FIG. 12) is provided on the base substrate 60. The gate insulating film 57 is formed so as to cover the gate electrode 62. A semiconductor layer 65 and n.sup.+ Si layers 67 and 68 are formed on the gate insulating film 57. On the thus-obtained layers, the source electrode 63 protruding from the above-described source line 53 and a drain electrode 64 which is integrated with the pixel electrode 54 are formed.
Still referring to FIG. 13, a counter electrode 71 is formed on a counter substrate 70 so as to face the pixel electrode 54. A shielding film 73 is provided so as to face the TFT 51. The base substrate 60 and the counter substrate 70 with alignment films (not shown) are disposed in an opposing manner with a liquid crystal layer 80 interposed therebetween, thereby forming a liquid crystal display device. If necessary, a color filter may be provided where the counter electrode 71 is provided. As can be seen from FIGS. 12 and 13, since holes 55a are provided through the Cs electrode 55, the surface of the pixel electrode 54 which is formed thereon via the gate insulating film 57 reflects the holes 55a and becomes uneven.
In order to enhance the use of light efficiency of the liquid crystal display device shown in FIG. 13, the pixel electrode 54 needs to be placed as close to the gate line 52 and the source line 53 as possible, and be made as large as possible. However, at a portion where the pixel electrode 54 is close to the gate line 52 and the source line 53, a transverse electric field may cause reverse tilted domains within the liquid crystal material which leads to poor display quality, or may cause a frequent leakage failure.
The pixel electrode 54 and the source line 53 are formed so as to overlap the gate line 52 via the insulating film 57 in order to minimize the effects caused by the above-described problems. In this case, parasitic capacitance may occur at the overlapping portion of the gate line 52 with the source line 53 and the pixel electrode 54, thereby inducing display defects caused by, for example, cross-talk. Moreover, since the periphery of the pixel electrode 54 is straight, the display quality may be deteriorated (e.g., coloring of the reflected light) due to nonuniform wavelength characteristics of the device caused by light interference.